The effect of host genotype on incubation period, receptivity, lesion diameter and leaf area damage of Didymella arachidicola on nine peanut (Arachis hypogaea L.) genotypes was investigated under monocyclic infection in the glasshouse. The genotypes, Florunner. P 84/5/256, C 347/5/6, C 346/5/8 and P 105/3/7, resistant to the pathogen in field trials, had a longer incubation period, reduced receptivity, lesion diameter, and percentage leaf area damage, than susceptible genotypes. Among the susceptible genotypes, Tamnut 74 had the shortest incubation period, and highest receptivity, the largest lesion diameter, and percentage leaf area damage. The other susceptible genotypes. Egret, 38/7/20, and P 84/5/112, were intermediate for these variables. Production of pycnidia and pseudothecia of the pathogen could not be demonstrated in infected leaf tissues of any of the genotypes studied. There was significant interaction between plant age and disease development. Younger plants had a shorter incubation period, higher receptivity, larger lesion diameter, and percentage leaf area damage than older plants. Correlation coefficients among incubation period, receptivity, lesion diameter, and leaf area damage were highly significant. The possible role of these variables in disease epidemics and their use in glasshouse screening of peanut germplasm for resistance to D. arachidicola are discussed.